Flexible materials are increasingly under development for high performance applications. Examples of applications include inflatable structures, temporary shelters, protective apparel, and films for use as fluid barriers or selective membranes. In one example, lightweight inflatable habitats have been investigated for aerospace applications, such as lunar or other extraterrestrial stations. These structures may include a large flexible membrane that is inflated with a breathable atmosphere, providing an internal living space and defining the overall shape of the structure. Inflatable habitats provide several advantages when compared to conventional rigid structures, such as high strength-to-weight ratio, better damage resistance, and lower manufacturing costs (D. Cadogan et al. Acta Astronautica 44, 399 (1999)).
Flexible materials tend to undergo failure due to punctures or tears. This is in contrast to rigid engineering plastics, which tend to experience fatigue or brittle failure. For example, inflatable habitat materials can be damaged by small punctures or tears in the flexible outer layer, including damage due to micrometeoroid and orbital debris (MMOD).
Failure in flexible materials can be especially problematic. Since these materials typically are used to provide flexibility to a system, they may be subjected to repeated stresses during use. Once a puncture or tear is initiated, it may grow quickly if the material is subjected to large and/or frequent stresses. If the puncture or tear is healed by the application of another polymer to the failure site, the properties of the healed material may be impaired if the mechanical properties of the new polymer and the original material are not closely matched.
Previous attempts at healing puncture damage have focused on ionomers and space-filling gels. A self-healing response in an ionomer can be initiated by the transfer of energy when punctured by a fast moving projectile, typically a few millimeters in diameter. Frictional heating of the material from the passage of the projectile can contribute to reorientation of the polymer chains in the ionomer. This rearrangement can, under some conditions, seal the hole generated by the projectile. However, this healing occurs only when the damaged area is heated to near the melt temperature of the material (Kalista, S. J. Mechanics of Advanced Materials and Structures, 14, 391 (2007)).
A water-saturated space-filling gel has been proposed for self-healing vehicle tires (Nagaya, K. et al. JMSE International Journal, 49, 379 (2006)). In this system, the polymeric gel is bonded between two layers of rubber on the inner surface of a tire and is then saturated with water. Upon puncture, the saturated gel can expand and fill the puncture, sealing the leak. However, a relatively thick polymer layer (4 mm) is required to effectively seal nail puncture damage for typical tire pressures of 0.25 MPa.
It is desirable to provide a flexible material that can self-heal when subjected to a puncture or tear. Ideally, the flexible material can autonomically self-heal, without the need for manual intervention such as heating. The self-healing response may prevent the loss of fluid contained by the flexible material. It is also desirable to provide a flexible material that can maintain most or all of its advantageous properties after it has self-healed.